Rails Arel and Active Record predicates

Question

I might be lacking some understanding concerning Arel objects in Rails.

You can do something like:

u = User.arel_table

then and crazy query additions like:

u.join(:microposts)

(implicit .on(users[:user_id]) ).

But how do you convert an Arel relation to a list of Rails objects?

Update

A better example is this. I just finished Michael Hartl's Rails tutorial where he does something similar to this in the Micropost controller:

  followed_ids  = %(SELECT followed_id FROM relationships WHERE follower_id = :user_id)
  where("user_id in (#{followed_ids}) OR user_id = :user_id", :user_id => user)

I hate the raw SQL. So I want to refactor, so far the best I have is this:

followed_ids  = Relationship.select(:followed_id).where(:follower_id => user) 
where("user_id in (#{followed_ids.to_sql}) OR user_id = :user_id", :user_id => user)

I used to add .map(&:followed_id).join(", ") to the Relationship select which also works but then you have all ids in memory instead of a short select statement for the db to process.

My issue is that it seemed that I needed to do a .to_sql to feed to where and make it happy, when it has everything it needs to return me my objects. I am thinking of switching to Squeel which can do the subquery nicely but I want to understand Arel.

Answer 1

Well you can do

User.joins(User.arel_table.joins(:microposts))

But there's not much point in that since you can just do User.joins(:microposts) and get the same result. But if you have to create some more complex joins it could be handy.

Something like this can be useful though.

User.where(User.arel_table[:name].matches('John%'))

And remember while you can take advantage of some of the core Arel stuff in Rails, it's often better to stick to the simpler methods like .where(:name=>'John') or .where('name LIKE ?', 'John%') since Rails will turn them into Arel objects behind the scenes anyway.